[Developments in Petroleum Science] Geophysics for Petroleum Engineers Volume 60 || Fundamentals of Petroleum Geology

Chapter 2

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Developments in Petroleum Science, Vol. 60. http://dx.doi.org/10.1016/B978-0-444-

© 2013 Elsevier B.V. All rights reserved.

Fundamentals of PetroleumGeology

Chapter Outline
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2.1. Introduction

2.2. Formation of Organic

Matter

2.3. Origin of Petroleum

2.4. Occurrence of Petroleum

Systems

2.5. Sedimentation and

Deformation Process

2.6. Geologic Times

2.7. Petroleum Reservoirs

2.8. Hydrocarbon Reservoirs

2.8.1. Clastic Sediments

2.8.2. Chemical

Sediments

2.8.3. Source Rocks

2.9. Petroleum Traps

2.9.1. Anticline Trap

2.9.2. Fault Trap

2.9.3. Salt Dome Traps

2.9.4. Stratigraphic Traps

2.10. Traps Associated with

Fracture Networks

2.11. Reservoir Rocks

2.12. The Role of a Geologist

2.13. The Role of Geophysics

2.14. Exploration and

Appraisal Wells
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References 36

2.1 INTRODUCTION

Petroleum engineers are responsible for planning and executing the develop-

ment and production of petroleum reserves. In most cases, however, they

are usually not heavily involved with the discovery, delineation, and evalua-

tion of new oil and gas fields. Those tasks are normally carried out by the

geologists, geophysicists, and petrophysicists of an “Operating Company” in

its Exploration and/or Development Department.

In this chapter, we provide a brief overview of petroleum geology. We

begin with the formation of organic matter and the origin of petroleum.

We then discuss occurrence of petroleum systems, comprised of Source

Rock, Burial Depth and Temperature, Reservoir Rock, Migration Pathways,

Reservoir Seals, and Traps. Figure 2.1 depicts a petroleum system with its

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http://dx.doi.org/10.1016/B978-0-444-50662-7.00002-0

FIGURE 2.1 A petroleum system.

Geophysics for Petroleum Engineers16

different components. We elaborate on different types of petroleum traps

such as structural, salt related, and stratigraphic traps. We then discuss vari-

ous types of reservoir rocks such as clastic (sandstone and shale) and

carbonate rocks.

We conclude with geology, geophysics, and petrophysics, in connection

with reservoir geometry, volume, and assessment of reserves. In this section,

we discuss how geology combined with geophysical techniques defines the

geometry of a petroleum reservoir and how petrophysics is utilized to quantify

the reservoir quality and petroleum reserves.

2.2 FORMATION OF ORGANIC MATTER

With the notable exceptions of certain astronomers, most scientists, in the petro-

leum industry, contend that petroleum resources are primarily organic, in origin.

Certain types of organic matter formed at the Earth’s surface eventually

produce hydrocarbons. The process starts with photosynthesis in which plants

convert water and carbon dioxide to complex sugars (glucose) using the

energy of the sun. Glucose is the starting material for the synthesis of more

complex organic compounds either in plants or the animals that eat them.

Generally, most of the organic matter produced by photosynthesis is eventu-

ally returned to the atmosphere as carbon dioxide. Only about one CO2,

molecule in every million taken up by photosynthesis is converted to

Chapter 2 Fundamentals of Petroleum Geology 17

hydrocarbons. This recycling of CO2 is achieved by plant and animal respira-

tion and through oxidation and bacterial decay when organisms die. However,

the recycling of carbon as CO2 is not totally efficient in that a very small

amount (about 0.0001%) escapes and is buried.

Sediments, laden with dead (plant and animal) lake, or sea organisms are

heavier than water, and naturally deposit in the lower areas or basins under the

sea. These basins are originated by tectonic action and sea level changes.

When the sea level rises (relative to the base of a depositional basin), the sedi-

ments are buried deeper. As ocean basins gradually fill with layers of

sediments, the weight of the newer layers increases the pressure on the layers

below. This weight, or pressure at depth, along with heat, converts the organic

material to oil and gas.

The primary source of the organic matter that is ultimately transformed into

oil and gas are the remains of phytoplankton; microscopic floating plants such

as diatoms. The best environment for the accumulation of this organic matter is

in quiet waters such as a swamp, lake, or deep ocean basin. Here, the organic

matter can lie buried without being disturbed. However, to ensure its preserva-

tion and to prevent rapid decay, the water conditions need to be stagnant and

reducing (oxygen deficient or anaerobic) thus eliminating the possibility of aer-

obic bacterial decay or scavenging by fish, etc. Along with the organics, muddy

sediment also accumulates. Source rock starts life as an organic-rich mud, sub-

sequently to be converted to a claystone, shale, or marl.

2.3 ORIGIN OF PETROLEUM

Petroleum hydrocarbons are complex substances formed from hydrogen and

carbon molecules and sometimes containing other impurities such as oxygen,

sulfur, and nitrogen. They come in many combinations and types, from the

petroleum products used in cars and other internal combustion engines to nat-

ural gas used for heating and cooking. There are “light oils” and “heavy oils,”

wet gas and dry gas. However, what they all have in common is an origin

from organic matter; that is plants and small animals that were once alive that

have created the “source rock.”

“Source rocks,” the rocks that produce hydrocarbons, are rich in particular

types of organic matter. Chemical changes after burial convert plant and ani-

mal tissue to the complex molecules that eventually produce oil or natural gas

by the effects of heat and pressure on sediments trapped beneath the Earth’s

surface over millions of years. The ancient societies in Egypt, China, and

India made limited use of petroleum mainly as fuel for lamps, medicine,

and as caulking for boats and canoes. The modern petroleum age began a

century and a half ago. Advances in technology have steadily improved our

ability to find and extract oil and gas and to convert them to efficient fuels,

lubricants, and other useful consumer products.


2.4 OCCURRENCE OF PETROLEUM SYSTEMS

Petroleum systems occur in reservoirs within sedimentary basins—those areas

of the world where subsidence of the Earth’s crust has allowed the accumula-

tion of thick sequences of sedimentary rocks. Petroleum is composed of com-

pressed hydrocarbons and was formed millions of years ago in a process that

began when aquatic plant and animal remains were covered by layers of sedi-

ments (particles of rock and mineral). As bacteria and chemicals broke down

the organic plants and animal material, increasing layers of sediment settled

on top. Heat and pressure transformed the layers of sediment into sandstone,

limestone and other types of sedimentary rock, and transformed the organic

matter into petroleum. Tiny pores in the rock allow the petroleum to seep

in. These reservoir rocks hold the oil like a sponge, confined by other, low

permeability layers that form traps. For a rigorous definition and more on

petroleum systems see, for example, Magoon and Dow (1994).

2.5 SEDIMENTATION AND DEFORMATION PROCESS

Before we get into petroleum reservoirs, it is important to discuss the sedi-

mentation process with which much of the oil and gas reservoirs are asso-

ciated. We also want to briefly discuss different geologic time periods, how

different geologic structures are formed and how such structures have evolved

over millions of years. Figure 2.2 shows a picture of a geologic formation that

is visible to us (outcrop). A typical outcrop, such as the one depicted in

Fig. 2.2, contains a vast amount of information about many different tectonic

movements, sedimentation processes, uplifting, subsidence, deformation, and

other evolutionary natural events that geologists can uncover such historical

events through various types of modeling and testing of the hypothesis.

Subsequent chapters show how geophysical data can help geologists with

their analysis in building more reliable models.

The evolution of formation of geologic structures is accomplished

through careful analysis of natural processes, modeling, and various

FIGURE 2.2 A typical outcrop demonstrating different rock formations and starta. Black Dragon

Canyon, Utah Photo by Fred Aminzadeh.


hypotheses. For example, Fig. 2.3 shows different time frames in which a

“disconformity” is formed. At the top of the figure, going back several mil-

lion years, the sediments characterized by ABCD were deposited under the

sea bed. Then (the second model from the top), the uplift of the beds above

the sea level, caused by tectonic forces, expose them to erosion. Note the

erosion has stripped away sediment package D and part of C, creating an

irregular collection of hills and valleys (model 3 from above). Finally, at

the bottom model, we note creation of a new package of sediments marked

as E created from subsidence below the sea that is deposited on top of C.

The irregularity of C package is preserved as an unconformity. Later on,

we will see (e.g., when we discuss seismic attributes) how seismic data

and petrophysical information can help resolve different sediment packages

(chronostratigraphic units) and the corresponding unconformities.

FIGURE 2.3 A Schematic view of the deformation, uplift and subsidence process. From Levin

(2013). Reprinted with permission of John Wiley & Sons, Inc.


Similarly, different stages in the process of formation of the angular

conformity are demonstrated in Fig. 2.4.

Figure 2.5 shows another outcrop based on which geologists would inter-

pret and come up with a plausible assessment of how sedimentary breaks, or

“disconformities,” have evolved. Here, we can see the flat layers of rock

that at first glance look like continuous layering of sediment. The two

FIGURE 2.4 Formation of Angular conformity. From Levin (2013). Reprinted with permissionof John Wiley & Sons, Inc.

FIGURE 2.5 A picture of Grand Canyon depicting the unconformities and the associated geo-

logic time.


formations highlighted are associated with the 20 million year gap between

the “Redwall” and the “Supai” and the 150 million year gap between the

“Muav” and the “Redwall.” This can further be confirmed by looking at the

associated fossils that allow us to determine the ages of the rocks and deter-

mine the large gaps in the geologic times between the corresponding layers.

2.6 GEOLOGIC TIMES

It is important to recognize that geologic times are associated with different

sedimentation processes over millions of years, corresponding to different

“strata.” While due to the development of new dating methods and refinement

of previous ones, geologic time scales have gone through constant revisions,

the main geologic times are somewhat well established. Figure 2.6 shows a

FIGURE 2.6 A schematic view of geologic times.


typical geologic timetable: the approximately 4.6 billion years of the Earth’s

life span are divided into major intervals or geologic periods. For example,

much older formations (e.g., pre-Cambrian) are at the bottom of the geologic

time with an age range starting from 550 million years. The “younger” sedi-

ments (e.g., those from Cretaceous period, belonging to the Mesozoic era)

with an age range of 65–144 million years (aka Myrs) are near the top of

the geologic age range.

2.7 PETROLEUM RESERVOIRS

Oil and gas accumulations are result from the coincident occurrence of the

following six elements:

Source Rock

Burial depth and temperature

Reservoir Rock

Migration pathways

Seal Rock

Trap

Three-dimensional (3D) seismic surveys enable the geologist and geophysicist

to investigate many of these key elements—identifying likely migration paths,

inferring the relative timing of trap formation and charge and measuring the

geometry and size of closed structures. In some cases, reservoir quality and

even the presence of fluid hydrocarbons may be estimated. Rock physics is

a key component of analyzing the reservoir. Much of this is related to the

source rock, seals and the capacity of the reservoir to contain hydrocarbons.

Porosity is a key ingredient and will determine the supply of petroleum that

is contained in the rocks. Seismic velocity can be related to porosity. The

empirical Wyllie Time Average equation (Wyllie et al., 1956, 1958) relates

velocity to porosity by using the time average of acoustic (seismic) travel

through the rock matrix and the fluid-filled pores.

’wyl ¼1

Bcp

Dt�Dtma

Dtf �Dtma

� �, (2.1)

where ’wyl is the Wylie Time Average sonic porosity. Dt is the observed

interval transit time (inverse velocity). Dtma is the matrix (solid) interval tran-

sit time. Dtf is the fluid interval transit time. Bcp is an arbitrary constant used

to keep the Dt–’ relationship linear.

Wyllie’s equation is reasonably valid for consolidated sandstones but is

generally an oversimplification for unconsolidated sandstones and carbonates.

As a general rule, velocity decreases will accompany porosity increases, as

related by another empirical relationship by Raymer et al. (1980):


’RHG ¼RHG 1�Dtma

Dt

� �, (2.2)

where ’RHG is the Raymer–Hunt–Gardner sonic porosity. Dt and Dtma are as

above. RHG is an arbitrary constant (0.4<RHG<0.8).

For the formation of hydrocarbons within the basin: there must have been

a source rock, rich in organic carbon (a rock with abundant hydrocarbon–

prone organic matter 0.5–2% by weight).

For conversion of organic matter to hydrocarbons, there must have been suf-

ficient heat over long periods of time to convert the organic carbon into hydro-

carbons. The temperature for oil generation and maturation is 50–150 �C. Suchhigh temperatures are usually achieved at depths of between 2 and 4 km. Thus

the sedimentary basin will need to be deep enough to ensure that the source

rock reaches the required depth.

2.8 HYDROCARBON RESERVOIRS

Hydrocarbon reservoirs are rocks that have:

l Sufficient porosity (void space) to store commercial volumes of

hydrocarbons.

l Sufficient permeability (fluid flow capability) to be able to deliver the

hydrocarbons to extraction wells.

l Sufficient hydrocarbon saturation (volumes of hydrocarbons relative to

other fluids) to be an economic resource.

Since oil is lighter than water and gas is lighter than both, when a hydrocar-

bon reservoir is found, it is stratified with gas on top, oil in between, and

water on the bottom, if all three phases are present.

Sedimentary rocks fall into one of four basic groups. These are sandstones,

shales, carbonate rocks, and evaporites. These rocks are generated by two

principal processes:

1. Erosion, transport, and deposition of sediments, as well as

2. Chemical solution and precipitation.

The erosion process is one in which solid particles resulting from landweathering

are transported and usually deposited in water environments as sediments. The

solid materials result from complete weathering of igneous rocks. Sediments

accumulate as fragmented material and result in sedimentary deposits having a

clastic texture. As the sedimentary material is transported, abrasion processes

round the grains.

2.8.1 Clastic Sediments

Clastic sediments are predominantly clay minerals and quartz particles, with

minor amounts of Feldspars, micas, and heavy minerals. Porosity results from


the space between the grain particles that is not filledwith cement or clay. Porosity

is usually in the range from10% to 30%depending on the grain sizes, compaction,

and the amount of cement present between the pores. Permeability, which is the

property that permits fluid to flow through the pores, is controlled by the amount

of cement, the degree of compaction, and the magnitude and variation of

grain sizes.

2.8.2 Chemical Sediments

The second source of sedimentary deposits is a result of chemical precipita-

tion of solids from solution in water. Dissolved solids in surface waters also

are weathering products. Soluble salts are leached from rocks during

weathering and transported by flowing waters to quiet waters where they

are precipitated, by either organisms or evaporation. The Colorado River,

source of much irrigation and drinking water in Southern California and

Arizona, is notable in its unusually high dissolved solids content.

Limestones are formed by chemical precipitation of calcite (calcium car-

bonate) or by aggregation of preexisting calcite particles. The most common

sources of these particles are animal skeletons and plant secretions. The solu-

ble elements precipitating from water are primarily calcium, magnesium,

sodium, potassium, and silicon. The bulk of this chemical precipitation is

organism produced and referred to as biogenic chemical sediments. Nonbio-

genic chemical sediments are much less common and result from evaporation.

Chemical sedimentation results in a granular texture. The size of the particles

greatly influences the porosity and permeability of limestones. Extremely fine

particles result in a very dense, low permeability rock termed micrite.

Water flowing through the pores of a limestone can greatly change the texture

of the rock by leaching the grains to produce vugs. If interconnected, these vugs

result in locally high porosity and very high permeability. Both the porosity and

permeability of a limestone can be reduced if more calcite is deposited. Dolomite

may be formed if the water causes partial replacement of calcium by magnesium

to form magnesium-calcium carbonate. Such post depositional chemical process

is known as diagenesis. Dolomitization results in increased porosity because the

dolomite crystal unit cell is more compact than that of calcite, which it replaces.

The porosity and permeability of a bed can be greatly influenced by the degree

of dolomitization. Porosities of carbonate rocks range from 5% to 35% to even

large caverns such at the Yates field, in West Texas.

2.8.3 Source Rocks

Not all sedimentary rocks contain oil or gas. Oil and natural gas originate in

petroleum source rocks. Source rocks are sedimentary rocks that were depos-

ited in very quiet water, usually in still swamps on land, shallow quiet marine

bays, or in deep submarine settings. Source rocks are comprised of very small


mineral fragments. In between, the mineral fragments are the remains of organic

material, usually algae, small wood fragments, or pieces of the soft parts of plants

and animals.When these fine-grained sediments are buried by deposition of later,

overlying sediments, the increasing heat and pressure resulting from burial turns

the soft sediments into hard rock strata. If further burial ensues, then temperatures

continue to increase. When temperatures of the organic-rich sedimentary rocks

exceed 120 �C (250 �F), the organic remains within the rocks begin to be

“cooked” and oil and natural gas are formed from the organic remains and

expelled from the source rock. It takes millions of years for these source rocks

to be buried deeply enough to attain thesematuration temperatures and additionalmillions of years to cook (or generate) sufficient volumes of oil and natural gas to

form commercial accumulations as the oil and gas.

Petroleum is composed of hydrocarbons and was formed millions of

years ago in a process that began when aquatic plant and animal remains were

covered by layers of sediments (particles of rock and mineral). As bacteria

and chemicals broke down the organic plants and animal material, increasing

layers of sediment settled on top. Heat and pressure transformed the layers of

sediment into sandstone, limestone and other types of sedimentary rock, and

transformed the organic matter into petroleum. Tiny pores in the rock allowed

the petroleum to seep in. These “reservoir rocks” hold the oil like a sponge,

confined by other, very low permeability layers that form a “trap.”

2.9 PETROLEUM TRAPS

Most hydrocarbon molecules are lighter than water and unless impeded, they

rise toward the surface. For commercial accumulations of hydrocarbons in

reservoir rocks, there must have been migration pathways or avenues, in

rocks, through which hydrocarbons migrate from the source rock and, reach

a trap. A hydrocarbon trap is some geometrical configuration of very low per-

meability rocks (seals) in configurations, which halts further migration.

A seal rock keeps the oil entrapped in reservoir from migrating away.

Understanding the geological and geomechanical nature of seals has become

one of the vital issues for successful exploration and development efforts. Seal

rocks are very low permeability formations through which oil and gas cannot

move effectively—such as mudstone, silt, clay stone, and anhydrite.

There are many types of hydrocarbon trap mechanisms. Figure 2.7 shows

different types of trap with their associate geologic features.

We will highlight the following four basic forms of traps in petroleum

geology further:

l Anticline Trap

l Fault Trap

l Salt Dome Trap

l Stratigraphic Trap

l Traps with the Fracture Network.

FIGURE 2.7 Display of different types of hydrocarbon traps seals, source rocks and migration

paths of hydrocarbons generated in the source rocks. Courtesy of BG-Group. http://www.bg-

group.com/OURBUSINESS/OURBUSINESS/Pages/GeologyandGeophysics1.aspx.


The common link between the first three is simple: some part of the Earth

structures has moved in the past, creating a barrier (seal) to hydrocarbon flow.

Stratigraphic traps are result of sedimentation process.

2.9.1 Anticline Trap

An anticline is an example of rocks, which were previously flat, but have been

bent into an arch. Hydrocarbons that find their way into a reservoir rock that

has been bent into an arch will flow to the crest of the arch, and get stuck

(provided, of course, that there is a seal rock above the arch to keep the

entrapped hydrocarbons in place).

Figure 2.8 is a cross section of the Earth showing typical Anticline Traps.

Figure 2.9 shows a cross section of the seismic image of an anticline trap.

Reservoir rock that is not completely filled with oil also contains large amounts

of salt water. In most cases, such reservoirs also include a “gas cap” with the

associated gas forming under a seal on the top of the reservoir.

2.9.2 Fault Trap

Fault traps are formed by movement of rock along a fault line. In some cases,

the reservoir rock has moved opposite a layer of impermeable rock. The

Typical Anticline Formation

Shale Sandstone

Free-phasegas

Oil

WaterLimestone

FIGURE 2.8 A typical anticline trap. From the Department of Natural Resources, Lousiana

State Government: http://dnr.louisiana.gov/assets/TAD/education/BGBB/4/oil_anticline.gif.

FIGURE 2.9 Seismic imaging of the subsurface, showing an anticline trap.


impermeable rock thus prevents the oil from escaping. In other cases, the fault

itself can be a very effective trap. Movement along the fault surfaces gener-

ates a very fine-grained “rock flower,” or “gauge” within the fault zone which

is smeared as the layers of rock slip past one another. This very fine-grained

material has such low permeability that it can act as a trap to prevent further

hydrocarbon migration.

Figure 2.10 shows a cross section of rock showing a fault trap—in this

case, an example of gouge. This is because the reservoir rock on both sides

http://dnr.louisiana.gov/assets/TAD/education/BGBB/4/oil_anticline.gif

Water

Impervious shale

Impervious shale

Impervious shale

Impervious shale

Impervious shale

Water

Oilpool

FIGURE 2.10 A typical Fault trap. From http://www.kgs.ku.edu/Publications/Bulletins/6_1/

02_origin.html.


of the fault would be connected, if not for the fault separating the two. In this

example, it is the fault itself that is trapping the oil.

2.9.3 Salt Dome Traps

Salt domes and diapers buried kilometers below the surface of the Earth move

upward until they break through to the Earth’s surface, where they are then

dissolved by ground- and rainwater. In the subsurface under heat and pressure,

salt deposits will flow, plastically, much like a glacier that slowly but contin-

ually moves up. To get all the way to the Earth’s surface, salt has to push

aside and break through many layers of rock in its path. This is what ulti-

mately will create the oil trap.

As is shown in Fig. 2.6, salt has moved up through the Earth, punching

through and bending rock along the way. Oil can come to rest right up against

the salt, which makes salt an effective seal rock. In the Niger Delta and other

very recent rapid depositional areas, shales will also move plastically and

form Shale Diapers, much like salt diapers. Also shown in Fig. 2.11 is a sche-

matic view of a seismic survey at the top (to be discussed in detail in

Chapter 3) and a well drilled through the salt body.

In recent years, through the recent advances made in seismic technology,

we are also able to see many prolific subsalt reservoirs (such as those seen

in the Gulf of Mexico, offshore Brazil, and offshore West Africa). This was

not possible in the past due to the very absorptive nature of salt, which rapidly

absorbs seismic wave energy. Figure 2.12 shows an example of a Gulf of salt

structure.

http://www.kgs.ku.edu/Publications/Bulletins/6_1/02_origin.html

http://www.kgs.ku.edu/Publications/Bulletins/6_1/02_origin.html

FIGURE 2.11 An example of a salt body trap.

FIGURE 2.12 A Gulf of Mexico subsalt model. Courtesy of Union Oil/Bakersfield Muesuem of

Art.


2.9.4 Stratigraphic Traps

The second major class of oil trap in petroleum geology is the stratigraphic

trap. It is related to sediment deposition or erosion and is bounded on one

or more sides by zones of low permeability. Because tectonics ultimately con-

trols deposition and erosion; however, few stratigraphic traps are completely

without structural influence. There are many types of stratigraphic traps. Some

are associated with the many transgressions and regressions of the sea that have

occurred over geologic time and the resulting deposits of differing porosities.

Others are caused by processes that increase secondary porosity, such as the

dolomitization of limestones or the weathering of strata once located at the

Earth’s surface. Figure 2.13 shows an example of stratigraphic trap.

Stratigraphic traps are analyzed using the concepts of sequence stratigra-

phy which is the study of the origin, relationship, and extent of rock layers

(strata). With the introduction of seismic technology, yet a newer discipline

in geology was established in the sixties, called seismic sequence stratigraphy.

This was aimed at utilizing seismic data to better define and understand dif-

ferent types of stratigraphic traps (onlap, offlap, toplap, etc.) and better map

and analyze different sequence boundaries. Many seismic attributes (e.g.,

instantaneous phase) were introduced to better highlight different stratigraphic

features (more on this in Chapter 3). Figure 2.14 shows an example of a 3D

sequence stratigraphy analysis combining the seismic and well log data. The

process involves transforming the conventional seismic section (in black

and white) to its equivalent geologic time (through Wyler transformation)

(in color). The ultimate goal of this process is to create seismic sections that

are more directly related to the geologic times and the corresponding strata

as described earlier.

FIGURE 2.13 A stratigraphic trap with permeability seal.

FIGURE 2.14 Calibrating chronostratigraphy with absolute geological age using seismic and

well log data. Courtesy of dGB Earth Sciences.

FIGURE 2.15 Formation of vertical fractures from stress relief (Wyrick and Borchers, 1981).


2.10 TRAPS ASSOCIATED WITH FRACTURE NETWORKS

With the recent increased interest in shale gas and liquid (oil) shale reservoirs,

it is important to discuss the traps that are associated with fractures. As shown

in Fig. 2.15, due to different compressional and shear stress, natural fractures

are created inside the rocks. In some cases, such fracture networks (e.g., in the

case of shale gas and shale oil reservoirs) become traps for hydrocarbons. Of

course, in many situations processes to further stimulate these fractures to

improve permeability and thus increase production, such as hydraulic


fracturing, are introduced. In Chapter 9, we will further discuss this topic and

introduce yet another emerging geophysical method called passive seismic or

micro-earthquake (MEQ) data. We will show how MEQ data can be used both

to help image and analyze the natural fractures and help design a more effec-

tive hydraulic fracturing treatment.

2.11 RESERVOIR ROCKS

Oil and gas reservoir rocks are porous and permeable (Leverson). They con-

tain interconnected passageways of microscopic pores or holes that occupy

the areas between the mineral grains of the rock. The oil and natural gas that

are produced from oil and gas fields reside in porous and permeable rocks

(reservoirs) in which these liquids have collected and accumulated throughout

the vast expanse of geologic time.

Porosity is a measure of the spaces within the rock layer compared to the

total volume of rock. Porosities are measured in percentages with the average

reservoir ranging from 7% to 40%. Though both are porous, a sponge is much

more porous than a brick, and though both can hold water in their pores, the

sponge has a much higher capacity for holding liquids. The net rock volume

multiplied by porosity gives the total pore volume: that is, the volume within

the sedimentary package that fluids (hydrocarbons and water) can occupy.

Figure 2.16 shows an example of a porous reservoir rock.

Permeability is a measure of how well liquids and gases can move through

the rock and, thus, is a function of how well the pores within the rock are

connected to each other. It is measured in units named Darcy (D). Typical res-

ervoir permeabilities range from mD to tens of D and can vary throughout

each reservoir, depending upon the type of reservoir and its origin.

FIGURE 2.16 Photomicrograph of reservoir rocks showing the porosity and connected pores

(i.e., permeability).


2.12 THE ROLE OF A GEOLOGIST

Geology is mostly an observational and intuitive science. The geologist works

much like a detective, arriving at the scene of a crime millions of years after it

was committed, with many of the clues having been destroyed and/or altered

over time. Based on incomplete and sparse observations, the geologist must

create a viable model of the subsurface, consistent with geologic principles,

upon which an exploration and/or development program can be based. Many

of the observations available to the geologist can be made by direct surface

observations or airborne and/or satellite images. Figure 2.17 shows an air-

photo mosaic of the breached Circle Ridge Anticline, Wyoming. Erosion

has stripped off the overburden and upper layers of the structure, exposing

the core and allowing geologists to infer what lies below.

Using surface-based studies of geologic relationships between the modern

environments to interpret the subsurface? The present is the key to the past.

Uniformitarianism and Hutton’s principles are not perfect to help geologists

make informed interpretations about relationships and predictions about subsur-

face systems. So, while we are often faced with the challenge of incomplete and

discontinuous data about a specific subsurface system, we can also leverage our

knowledge about geologic systems and relationships that control how litholo-

gies are produced, structural elements evolve, and fluids and other pore-filling

media evolve over time to predict and interpret what occurs in the subsurface.

Figure 2.18 shows an elevation map based on geologic interpretation of

the surface geology, well control (subsurface geology), and seismic data. Such

geologic maps were especially useful before the age of 3D visualization

where geologists could mimic a 3D earth on a 2D map where different

FIGURE 2.17 Air-photo mosaic showing surface of circle ridge anticline. After Landes (1970).

Courtesy of John Wiley and Sons.

FIGURE 2.18 Phosphoria formation structural map, circle ridge anticline. After Landes (1970).

Courtesy of John Wiley and Sons.

FIGURE 2.19 SW–NE structural cross section through the circle ridge anticline. After Landes(1970). Courtesy of John Wiley and Sons.


“contours” represent different “elevations.” Naturally, the contours are

truncated when a geological fault is encountered.

Figure 2.19 shows an SW–NE structural cross section through the anticline,

based upon surface geology, well control (subsurface geology), and seismic data.


2.13 THE ROLE OF GEOPHYSICS

Other geologic observations must be interpreted via indirect geophysical

measurements. Geophysical measurements are Earth physical property mea-

surements via satellite, airborne, surface, marine, and/or borehole instrument

packages.

While marine, surface, and airborne gravity and magnetic surveys are

often utilized for regional studies, the most commonly utilized surface and

marine geophysical techniques are seismic reflection seismology. In this

technique, seismic (acoustic) echoes from explosive or vibratory, and/or

acoustic sources are utilized to develop subsurface models for geologic

interpretation. This technique allows the geologist to image features, in

three dimensions, which cannot be directly seen, much like ultra sound

imaging allows physicians to noninvasively image the human body and/or

materials and structural engineers to nondestructively image the interiors

of complex structures.

The structural contour map of Fig. 2.18 and structural section of Fig. 2.19

are based on subsurface (well control) geology and seismic data, as well as

surface geology.

2.14 EXPLORATION AND APPRAISAL WELLS

An exploration well drilled on acreage with no known petroleum reserves or

production is known as a Wildcat Well. This name traces its origin to the early

days of the oil industry, when promoters would drill wells (usually with other

people’s money), based upon little more than a dream, a hunch, or simply

because they owned the mineral rights. The costs of drilling modern explora-

tion wells is so great, however, that Wildcats are seldom drilled without

developing detailed geological and geophysical models, first. In fact, many

modern E&P organizations will spend the equivalent of the costs for several

wells prior to drilling a single wildcat, because the preliminary work provides

a three-dimensional model, while a well only provides information about that

particular location.

If the results of a wildcat well are promising, appraisal wells are drilled to

further assess the quality, distribution, and extent of the reservoir. Wireline or

logging while drilling (LWD) logs are used to calculate the proportion of the

sedimentary packages that contains reservoir rocks. The bulk rock volume

multiplied by the net-to-gross ratio gives the net rock volume of the reservoir.

Detailed evaluations of wireline logs or formation evaluation (Chapter 4) are

used to estimate the amount of hydrocarbons in place.

The transition from what can be measured to what is desired requires

either statistical or deterministic petrophysical models. The deterministic

models, however, are really empirical models based on small numbers of lab-

oratory measurements.


REFERENCES

Harold L. Levin, The Earth Through Time, 10th Edition, August 2013, © 2013.

Landes, K.K., 1970. Petroleum Geology of the United States. Wiley-Interscience, New York.

Magoon, L.B., Dow, W.G. (Eds.), 1994. The Petroleum System, from Source to Trap: AAPG

Memoir 60.

Raymer, L.L., Hunt, E.R., Gardner, J.S., 1980. An Improved Sonic Transit Time to Porosity

Transform: SPWLA 21st Annual Logging Symposium.

Wyllie, M.R.J., Gregory, A.R., Gardner, G.H.F., 1956. Elastic wave velocities in heterogeneous

and porous media. Geophysics 21 (1), 41–70.

Wyllie, M.R.J., Gregory, A.R., Gardner, G.H.F., 1958. An experimental investigation of factors

affecting elastic wave velocities in porous media. Geophysics 23 (3), 459–493.

Wyrick, G.G., Borchers, J.W., 1981. Hydrologic effects of stress-relief fracturing in an Appala-

chian Valley. U.S. Geological Survey Water-Supply Paper 2177, 51 p.

http://refhub.elsevier.com/B978-0-444-50662-7.00002-0/rf0005





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[Developments in Petroleum Science] Geophysics for Petroleum Engineers Volume 60 || Fundamentals of Petroleum Geology